Had a bitch if a time getting my bulkhead sleeve into the 15mm insertion range so I crafted this from a few spare parts

1/2” x 12” Carrier bolt
2” fender washer
(2) 1/2 nuts

With this you can apply enough pressure by hand or hammer from the rear side of the truck

Wasn’t getting it in far enough without this

scissors for size reference

 

I just used the rubber end of a hammer. The hammer head makes a good grip to push with.